The disclosure of Japanese Patent Application No. 2000-018688 filed on Jan. 27, 2000 including the specification, drawings and abstract is incorporated herein by reference in its entirety.
1. Field of the Invention
The invention relates to a shift control apparatus for a continuously variable transmission of a motor vehicle. In particular, the invention is concerned with shift control of a continuously variable transmission when an electronic throttle valve is faulty.
2. Description of the Related Art
There is known a shift control apparatus for a motor vehicle including (a) an internal combustion engine whose output is controlled by an electronic throttle valve for adjusting the flow rate of intake air, and (b) a continuously variable transmission disposed in a power transmission path between the engine and drive wheels, for changing the speed ratio. The shift control apparatus is adapted to control the speed ratio of the continuously variable transmission in accordance with operating or driving conditions of the vehicle. In general, shift control of the continuously variable transmission is performed according to predetermined shift conditions in the form of, for example, a map, using the output requirement indicated by the driver, such as an accelerator operation amount, and the vehicle speed as parameters. The shift conditions are usually determined such that the speed ratio (=the input-side rotation speed/the output-side rotation speed) increases with an increase in the output requirement and with a reduction in the vehicle speed.
When the electronic throttle valve fails, the output of the engine cannot be controlled with the opening of the throttle valve being maintained at a certain degree, but it has been proposed to perform special shift control that enables xe2x80x9climp-homexe2x80x9d, namely, permits the vehicle to limp or travel to a nearby repair shop, or the like. For example, an apparatus as disclosed in Japanese Patent No. 2616154 is adapted to perform shift control solely based on the output requirement made by the driver (i.e., the power demanded by the driver) when the throttle valve is faulty.
If shift control is performed solely based on the output requirement, and if the output requirement is kept large when the driver continues to depress the accelerator pedal in an attempt to increase the vehicle speed, for example, the rotation speed of the engine increases as a result of an increase in the speed ratio of the continuously variable transmission. As a result, the torque, and accordingly the output, are reduced, which produces a reduction or deterioration in the running performance (driving torque). Namely, even if the valve opening of the electronic throttle valve is kept constant, the output of the engine changes in accordance with the rotation speed, as shown in FIG. 7 by way of example, such that the engine output is gradually reduced after it reaches its maximum at a certain engine speed (NEP in FIG. 7). Thus, even if the torque is amplified in accordance with the speed ratio of the continuously variable transmission, a sufficiently large driving torque may not be obtained.
In the meantime, where a brake booster is provided for assisting the braking force by use of the intake manifold negative pressure of the engine, the intake manifold negative pressure is reduced when the electronic throttle valve fails and is maintained at a predetermined valve opening (e.g., when xcex8th=xcex8thF), as compared with the case where the electronic throttle valve is completely closed (xcex8th=0%), as shown in FIG. 6. Also, in this case, the intake manifold negative pressure tends to be reduced with a reduction in the engine speed (NE). Thus, if the rotation speed of the engine is reduced in accordance with a reduction in the vehicle speed when the vehicle is stopped, for example, a required booster pressure (e.g., the minimum booster pressure PB) cannot be obtained, with the result that the brake assist force is reduced, and the driver feels uncomfortable when applying a brake to the vehicle.
Here, it is to be understood that the magnitude of the negative pressure is considered being opposite to that of the positive pressure. Namely, the reduction in the negative pressure means that the pressure increases to be close to the atmospheric pressure (at which the negative pressure is equal to zero).
It is an object of the invention to provide a shift control apparatus that performs shift control on a continuously variable transmission, while avoiding reductions in the driving torque or the assist force of a brake booster due to a failure in an electronic throttle valve, and thus preventing the driver from feeling uncomfortable due to the reduction in the torque or brake assist force.
To accomplish the above and other objects, the invention provides a shift control apparatus for a motor vehicle including an internal combustion engine whose output is controlled by an electronic throttle valve for adjusting the flow rate of intake air, and a continuously variable transmission disposed in a power transmission path between the internal combustion engine and drive wheels and operable to change a speed ratio thereof, comprising a shift control device that controls the speed ratio of the continuously variable transmission depending upon operating conditions of the vehicle such that an input-side rotation speed of the transmission is variable within a first range during normal running of the vehicle; a fault detecting unit that determines whether the electronic throttle valve is faulty or not; and a restricting unit that restricts the speed ratio of the continuously variable transmission such that the input-side rotation speed of the transmission is variable within a second range when the fault detecting unit determines that the electronic throttle valve is faulty, the second range being set to be narrower than the first range.
In one preferred form of the invention, the restricting unit comprises an upper-limit setting unit that sets an upper limit of the input-side rotation speed of the continuously variable transmission and restricts the speed ratio of the transmission so that the input-side rotation speed does not exceed the upper limit when the electronic throttle valve is faulty.
In the above form of the invention, since the speed ratio is restricted so that the input-side rotation speed does not exceed the upper limit when the electronic throttle valve is faulty, the rotation speed of the engine is also controlled to be equal to or lower than a predetermined value that corresponds to the upper limit of the input-side rotation speed. If the upper limit of the input-side rotation speed is set in the vicinity of a rotation speed at which the output of the internal combustion engine reaches its maximum, an excessive increase in the engine speed and a resulting reduction in the output can be advantageously prevented. Thus, the upper limit of the input-side rotation speed can be suitably determined in view of the problems associated with a failure in the throttle valve, thereby to cause the engine to operate at desired operating conditions.
In another preferred form of the invention, the restricting unit comprises a lower-limit setting unit that sets a lower limit of the input-side rotation speed of the continuously variable transmission, and restricts the speed ratio of the transmission so that the input-side rotation speed does not fall below the lower limit when the electronic throttle valve is faulty.
In the above form of the invention, since the speed ratio of the transmission is restricted so that the input-side rotation speed does not fall below the lower limit when the electronic throttle valve is faulty, the engine speed is also controlled to be equal to or higher than a predetermined value corresponding to the lower limit of the input-side rotation speed. If the lower limit of the input-side rotation speed is set in the vicinity of a rotation speed at which a certain intake manifold negative pressure required for operating the brake booster is obtained by an operation of the engine, an excessive reduction in the engine speed and a resulting reduction in the booster pressure can be advantageously prevented. Thus, the lower limit of the input-side rotation speed can be suitably determined in view of the problems associated with a failure in the throttle valve, thereby to cause the engine to operate at desired operating conditions.